Directional microphone with substantially frequency-independent directional characteristics



May 20, 1969 o. HOFFMANN 3,444,955 DIRECTIONAL MICROPHONE WITH SUBSTANTIALLY FREQUENCY- INDEPENDENT DIRECTIONAL CHARACTERISTICS Original Filed April 26, 1965 Sheet of 2 favsl r/ w r y 200 craes wee J'rcwa mic/3 5'00 cfcce': P60 Jae-0M0 nocrue P62 rscolvp ruse A5416 77/ IA! Cm INVENTOR 0 H BY Mala! W (57 0 ATTORNEY United States Patent Int. (:1. ohm 13/00 US. Cl. 18131 14 Claims ABSTRACT OF THE DISCLOSURE A directional microphone including a mechanical-electrical transducer and a single directional tube for directing sound to this transducer. This tube is positioned with one end adjacent the transducer and havng at the opposite end opening means adapted to be located closest to the source of sound. The tube is provided with l ngitudinally extending acoustic resistance means formed in the wall of the tube and having an acoustic resistance of a particular type.

Cross-reference to related application The present application is a continuation of my U.S. patent application Ser. No. 450,714 filed Apr. 26, 1965, entitled Directional Microphone with Substantially Frequency-Independent Directional Characteristics, now abandoned.

The present invention relates to a directional microphone with substantially frequency-independent directional characteristics. More particularly, the invention relates to a directional microphone which utilizes a single directional or acoustc tube to direct sound to the transducer unit.

Known directional microphones attempt to achieve frequency-independent directional characteristics by various constructions. In known constructions, the directional microphone comprises an acoustic tube having laterally extending acoustic slots, slits or openings covered with an acoustic resistive material. In one known construction, the length of the directional or acoustic tube is determined for the lowest frequency at which a specific directional characteristic is desired. The acoustic resistive covering is staggered or varied in dependence upon the length of the directional or acoustic tube in a manner whereby the tube becomes acoustically shorter as the frequency increases. Such a microphone is frequency-dependent. The directional characteristic increases with the frequency.

If, in this construction, the directional tube has a constant diameter, a covering having a specific acoustic resistance must be utilized if the sound reception along the entire length of the tube is to be constant. The ac ustic resistance of the covering increases as it approaches the microphone or mechanical-electrical transducer unit. Microphones of the aforementioned constructions are described in Radio Mentor, Volume 11, 1956, in an article entitled The Telemicrophone by Hans Joachim Griese; Radio Mentor, 1959, pages 090 to 093, in an article entitled A Telemicrophone with Frequency-Independent Directional Characteristic by Griese and Holfmann; United States Patent No. 2,856,022; and Acoustical Reports Acoustica, Volume 1, 1954, in an article entitled A Novel Microphone of Great Directional Selectivity by K. Tamm and G. Kurtze.

In the afore'described construction, either the part of the directional or acoustic tube closest to the source of sound and farthest from the transducer unit, or the part of the tube farthest from the source of sound and closest to the transducer unit is ineffective for high frequencies. Furthermore, in this type of construction, the directional tube is very long and its sensitivity is considerably lessened relative to the sensitivity of the transducer unit.

In another known construction, a bundle of directional or acoustic tubes of different lengths is utilized. This construction is described in Journal of the Acoustical Society of America,, No. 10, 1939, page 206, in article by W. P. Mason and R. N. Marshall.

In Proceedings of the I.R.E., July 1939, pages 438 to 455, H. F. Olson, in an article entitled Line Microphones, describes a microphone comprising a bundle of directional or acoustic tubes which are bent sharply in substantially V-shape configuration. This delays sound entering the tubes in its transmission time through said tubes and the bundle of said tubes thus functions as if it were longer than it is. This permits a reduction in length of the acoustic tubes. This construction is, however, awkward and diflicult to handle because of its 'unwieldiness and is expensive and difficult to manufacture. Furthermore, the directional tubes cause considerable losses and high damping at high frequencies and considerably lessen the sensitivity of the unit as a whole.

The principal object of the present invention is to provide a new and improved directional microphone.

An object of the present invention is to pro-vide a directional microphone with substantially frequency-independent directional characteristics.

Another object of the present invention is to provide a directional microphone which avoids and overcomes the difficulties and disadvantages of known directional microphones.

Another object of the present invention is to pr vide a directional microphone which utilizes a single, short directional or acoustic tube which functions with the effects of a long tube.

Another object of the present invention is to provide a directional microphone which has a greater sensitivity than known directional microphones.

In accordance with the present invention, a directional tube for directing sound to the mechanical-electrical transducer of a directional microphone is positioned with one end adjacent the transducer. The directional tube comprises an open opposite end spaced from the one end for receiving sound and a longitudinally extending acoustic resistance formed in the wall of the directional tube and having acoustic resistance increasing longitudinally from the open toward the one end of the directional tube. The acoustic resistance has a structure and configuration for providing determined sensitivity and phase shifting characteristics to produce a high directional effect and substantial frequency-independence of the microphone. The sensitivity decreases from a maximum level at the open end of the directional tube to a minimum level at a determined longitudinal distance from the open end. Sound in the directional tube is increasingly shifted in phase as it traverses the directional tube from the open end of the directional tube to the transducer.

In order that the present invention may be readily carried into effect, it will now be described with reference to the accompanying drawings, wherein:

FIG. 1 is a side view of an embodiment of the directional microphone of the present invention;

FIG. 2 is a graphical illustration of the sensitivity of the microphone of the present invention versus the length of the directional tube of said microphone;

FIG. 3 is a graphical illustration of the phase shift of the sound in the directional tube of the microphone of the present invention;

FIG. 4 is a graphical illustration of the acoustic resistance of the woven cover material of the directional tube of the microphone of the present invention; and

FIG. is a side view of a modification of the directional tube of the embodiment of FIG. 1.

In FIG. 1, a directional or acoustic tube 1, of short length, directs sound to a mechanical-electrical transducer unit 2. The acoustic tube 1 may have a length, for example, of 40 cm. The transducer unit 2 may comprise any suitable mechanical-electrical transducer such as, for example, a variable capacitance unit. The transducer unit 2 comprises a diaphragm or membrane 3 of known type which is schematically indicated by broken lines.

The housing of the transducer unit 2 includes an aperture 4 formed therethrough. The aperture 4 formed through a wall of the housing permits sound to be admitted to said housing and to be directed to the diaphragm 3 from the opposite side thereof from which the sound is directed via the directional tube 1. The directing of sound to the diaphragm 3 to the side thereof closer to the aperture 4 via said aperture, or even via several apertures (not shown), produces a directional effect which is of cardioid or supercardioid characteristic at low frequenones.

The directional tube 1 has an open end 5 which constitutes opening means at its end closest to the source of sound and farthest from the transducer unit 2. A longitudinally or laterally extending slot, slit or opening 6 and a longitudinally or laterally extending slot, slit or opening 7 are formed through the wall of the acoustic tube 1. The

circumferential width of the slot 6 increases by a small amount in the direction of the length of the acoustic tube 1 as it extends from the transducer unit 2 to the end 5 thereof at the source of sound. The circumferential width of the slot 7 increases by a considerably larger amount in the direction of length of the acoustic tube 1 as it extends from the transducer unit 2 to the end 5 thereof at the source of sound. The slot 6 extends longitudinally at a slight taper from the transducer unit 2 to the slot 7 and the slot 7 extends longitudinally at a relatively greater taper from the slot 6 to the end 5 of the tube 1.

The slot 6 is covered for most of its length 6a, with material 8 such as, for example, Perlon or silk, having a determined acoustic resistance per unit area. The cover material 8 may comprise any suitable material, of course. The remaining portion 612, of the length of the slot 6 is covered with material 9, having a determined acoustic resistance per unit area which is smaller than that of the cover material 8. The cover material 9 may comprise any suitable material, of course. The cover material 9 covers a portion 7a, of the slot 7 as well as the remaining portion 6b, of the slot 6.

The remaining portion 7b, of the length of the slot 7 is covered with material 11, having a determined acoustic resistance which is smaller than that of the cover material 9 and which is therefore smaller than that of the woven cover material 8. The woven cover material 11 may comprise any suitable material. The slots 6 and 7 are thus covered with three zones 6a, 6b and 7a and 7b of cover material 8, 9 and 11, respectively, of different acoustic resistance.

FIG. 2 illustrates the sensitivity in decibels of the directional microphone of the present invention for different frequencies in relation to the length of the directional or acoustic tube 1. The length of the directional tube is measured in the longitudinal or lateral direction from the open end 5 thereof. As illustrated by the curves of FIG. 2 for frequencies of 200 cycles per second, 500 cycles per second and one kilocycle per second, with the abscissa being the length of the tube in cm. and the ordinate being the sensitivity or amplitude of the microphone in decibels, the maximum sensitivity is at the open end 5 of the tube due to the greatest circumferential width of the cover material 11 being at said open end and due to the relatively small acoustic resistance of said cover material. The sensitivity of the microphone then decreases rapidly, due to the rapid narrowing of the circumferential width of the cover material 11, to a minimum level and then remains substantially constant for the greatest portion of the length of the tube. At the lower frequencies, the abrupt decrease in sensitivity from a maximum level to a minimum level is much more pronounced than at the higher frequencies, where the difference in sensitivity between the maximum and minimum levels is considerably less than at the lower frequencies.

Due to the configuration of the slots 6 and 7 and the cover material 8, 9 and 11 and due to the acoustic resistance characteristics of said cover material, in accordance with the present invention, sound entering the directional tube 1 at the open end 5 thereof is amplified and shifted in phase as it progresses in its transmission through said tube. The phase shift is greater and more pronounced at the lower frequencies than at the higher frequencies, as illustrated in FIG. 3, wherein the abscissa indicates the length of the acoustic tube in cm. beginning from the open end 5 and the ordinate indicates the phase shift /w in degrees. The phase shifting of the sound provides a delay in the time of transmission of the sound through the acoustic tube 1. The tube 1 thus functions as if it were considerably longer than it is.

The phase shift illustrated in FIG. 3 is indicated for frequencies of 200 cycles per second, 500 cycles per seccond, one kilocycle per second and 3 kilocycles per second, respectively. The phase shift is increased at a substantially linear constant rate at frequencies of one and 3 kilocycles per second. The phase shift is most pronounced and increases more abruptly at a frequency of 200 cycles per second. Thus, at the lower frequencies the sound is delayed longer in its transmission through the directional tube 1 than it is at the higher frequencies. At the higher frequencies, however, the sound is not as strongly affected.

The acoustic resistance of the directional tube 1 increases as said tube extends from its open end 5 to the transducer 2, as illustrated in FIG. 4, wherein the abscissa indicates the length of said tube in cm. from said open end and the ordinate indicates the acoustic resistance of the cover material of said tube in decibels per unit area.

At low frequencies, the frequency dependence of the microphone of the present invention is reduced to such an extent that said microphone is substantially frequencyindependent. This is due to the extremely rapid change of sensitivity of the microphone at its open end 5 from a high maximum level to a low minimum level, the sensitivity being very large at the open end 5 of the directional tube at low frequencies, and is also due to the more abrupt and pronounced increase in phase shift of the sound at low frequencies. This combination of sensitivity and phase shift characteristics provides a high direction effect and substantial frequency-independence at low frequencies.

FIG. 5 is a modification of the directional tube of the directional microphone of FIG. 1, wherein the slots 6 and 7 are replaced by spaced apertures of varying diameter corresponding to the circumferential width of the replaced portions of said slots and wherein the apertures are covered by cover material of different acoustic resistance corresponding to the cover material of the slots 6 and 7.

In FIG. 5, the directional or acoustic tube 1 may comprise a plurality of apertures which vary in diameter at a decreasing rate from the open end 5' of said tube to the mechanical electrical transducer (not shown). Thus, for example, in an operative embodiment of the modification of FIG. 5, two apertures 21, each having a diameter of 6.3 mm., may be formed through the wall of the directional tube 1' closest to the open end 5 thereof. Adjacent the apertures 21, two apertures 22, each having a diameter of 5.6 mm, y e f rmed through the wall of the directional tube 1. Adjacent the apertures 22, four apertures 23, each having a diameter of 4.2 mm., may be formed through the wall of the directional tube 1'. Adjacent the apertures 23, five apertures 24, each having a diameter of 3.5 mm., may be formed through the wall of the directional tube 1. Adjacent the apertures 24, eight apertures 25, each having a diameter of 2.8 mm., may be formed through the wall of the directional tube 1'. Adjacent the apertures 25, twenty apertures 26, each having a diameter of 2.5 mm., may be formed through the wall of the directional tube 1'.

In the operative embodiment of the modification of FIG. 5, the directional or acoustic tube 1' has a length of 41.6 cm. and a diameter of 15 mm. The apertures 21, 22, 23, 24, 25 and 26 are covered with cover material, not shown in the interest of maintaining the clarity of illustration, which corresponds in acoustic resistance to the cover material 8, 9 and 11. Thus, the cover material coverin the apertures 25 and 26 has the largest acoustic resistance and the cover material covering the apertures 21 and 22 has the smallest acoustic resistance per unit area. The acoustic resistance per unit area of the cover material covering the apertures 23 and 24 is smaller than that of the cover material covering the apertures 25 and 26 but larger than that of the cover material covering the apertures 21 and 22.

A desirable characteristic of the modification of FIG. is that it increases, rather than decreases, the sensitivity of the microphone. The sensitivity or amplification of the microphone utilizing the tube 1 is greater by approximately 6 decibels than that of a microphone without the directional tube in the frequency range up to about kilocycles per second. This increase in sensitivity also results from the back pressure on the membrane or diaphragm 3 due to the admission of sound into the transducer housing via the aperture 4. Such back pressure starts at approximately 150 cycles per second.

While the invention has been described by means of specific examples and in specific embodiments, I do not wish to be limted thereto, for obvious modifications will occur to those skilled in the art without departing from the spirit and scope of the invention.

What I claim is:

1. In a directional microphone including a mechanicalelectrical transducer, a single directional tube for directing sound to said transducer positioned with one end adjacent said transducer, said directional tube comprising a completely open opposite end adapted to be located closest to the source of sound and being spaced from said one end for receiving sound, and acoustic resistance means formed in the wall of said directional tube and having a structure and configuration for providing determined sensitivity and phase shifting characteristics to produce a high directional effect and substantial frequency-independence of said microphone.

2. In a directional microphone as defined in claim 1, wherein said acoustic resistance means formed in the wall of said directional tube extends in longitudinal di rection of the latter and has an acoustic resistance increasing longitudinally from said opposite toward said one end of said directional tube.

3. In a directional microphone including a mechanical-electrical transducer, a single directional tube for directing sound to said transducer positioned with one end adjacent said transducer, said directional tube having at the region of the opposite end opening means adapted to be located closest to the source of sound and being spaced from said one end for receiving sound, and longitudinally extending acoustic resistance means formed in the wall of said directional tube and having acoustic resistance increasing longitudinally from said opposite toward said one end of said directional tube, said acoustic resistance means having a structure and configuration for providing determined sensitivity and phase shifting characteristics to produce a high directional effect and substantial frequency-independence of said microphone, said sensitivity decreasing from a maximum level at said opposite end of said directional tube to a minimum level at a determined longitudinal distance from said opposite end.

4. In a directional microphone as defined in claim 3, wherein said acoustic resistance means formed in the wall of said directional tube extends in longitudinal direction of the latter and has an acoustic resistance increasing longitudinally from said opposite toward said one end of said directional tube, sound in said directional tube being increasingly shifted in phase as it traverses said directional tube from said opposite end of said directional tube to said transducer.

5. In a directional microphone as defined in claim 3, wherein said sensitivity decreases from a maximum level at said Opposite end of said directional tube to a minimum level at a determined longitudinal distance from said opposite end, sound in said directional tube being increasingly shifted in phase as it traverses said directional tube from said opposite end of said directional tube to said transducer.

6. In a directional microphone as defined in claim 3, wherein said acoustic resistance means formed in the wall of said directional tube has a structure and configuration for providing phase shifting of sound in said directional tube to delay the time of transmission of said sound through said directional tube.

7. In a directional microphone as defined in claim 3, wherein said acoustic resistance means formed in the wall of said directional tube has a structure and configuration for providing phase shifting of sound in said directional tube to delay the time of transmission of said sound through said directional tube, the phase shifting of said sound being greater at lower sound frequencies than at higher sound frequencies.

8. In a directional microphone as defined in claim 3, and including a housing, said mechanical-electrical transducer being positioned in said housing and having a diaphragm, wherein said acoustic resistance means formed in the wall of said directional tube has a structure and configuration for providing determined sensitivity and phase shifting characteristics to produce a high directional effect and substantial frequency-independence of such microphone at low sound frequencies, and an aperture formed in said housing for admitting sound to the side of said diaphragm opposite that adjacent said one end of said directional tube.

9. In a directional microphone as defined in claim 3, wherein said acoustic resistance means comprises a longitudinally extending slot formed through the wall of said directional tube and covering material covering said slot and having acoustic resistance increasing longitudinally from said opposite toward said one end of said directional tube.

10. In a directional microphone as defined in claim 9, wherein said slot has a circumferential width which varies from a maximum adjacent said opposite end of said directional tube to a width slightly greater than a minimum at a determined longitudinal distance from said opposite end and which varies from said slightly greater than minimum circumferential width to said minimum over the greater portion of the length of said slot, and covering material covering said slot.

11. In a directional microphone as defined in claim 9, wherein said slot has a circumferential width which varies from a maximum adjacent said opposite end of said directional tube to a width slightly greater than a minimum at a determined longitudinal distance from said opposite end and which varies from said slightly greater than minimum circumferential width to said minimum over the greater portion of the length of said slot, and covering material covering said slot and having acoustic resistance increasing longitudinally from said opposite toward said one end of said directional tube, said covering material comprising material having a plurality of longitudinally extending zones, each of said zones having an acoustic resistance dilferent from that of the others of said zones, the acoustic resistance of each of said zones increasing from a minimum in the zone adjacent said opposite end of said directional tube to a maximum in the zone adjacent said transducer.

12. In a directional microphone as defined in claim 3, wherein said acoustic resistance means comprises a longitudinally extending plurality of apertures formed through the wall of said directional tube and covering material covering said apertures and having acoustic resistance increasing longitudinally from said opposite toward said one end of said directional tube.

13. In a directional microphone as defined in claim 12, wherein said apertures have diameters which vary from a maximum adjacent said opposite end of said directional tube to a minimum at a determined longitndinal distance from said opposite end and which are said minimum over the greater portion of the length of said directional tube, and covering material covering said aperture.

14. In a directional microphone as defined in claim 12, wherein said apertures have diameters which vary from a maximum adjacent said opposite end of said directional tube to a minimum at a determined longitudinal distance from said opposite end and which are said minimum over the greater portion of the length of said directional tube, and covering material covering said apertures, said covering material comprising material having a plurality of longitudinally extending zones, each of said zones having an acoustic resistance difierent from that of the others of said zones, the acoustic resistance of each of said zones increasing from a minimum in the zone adjacent said opposite end of said directional tube to a maximum in the zone adjacent said transducer.

References Cited UNITED STATES PATENTS 2,856,022 10/1958 Kurtze et a1 181-05 2,939,922 6/1960 Gorike 179--121 3,095,484 6/1963 Beaverson et al. 179-1155 FOREIGN PATENTS 1,137,483 10/ 1962 Germany.

STEPHEN J. TOMSKY, Primary Examiner.

US. Cl. X.R. 179-1; 181--.5 

